In this explainer, we will learn how to identify and name esters and describe their physical properties.
Esters are organic compounds with the general chemical formula and the general displayed formula as shown below. An example of an ester would be methyl methanoate, which has the formula .
The functional group can be referred to as the ester link.
Ester molecules are organic compounds represented by the general formula and are formed by the reaction between alcohols and carboxylic acids.
Heating an alcohol with a carboxylic acid will produce an ester and water. This is called an esterification reaction, and it involves the alkoxide group of an alcohol replacing the hydroxyl group of a carboxylic acid.
In the equation below, the alkoxide group is represented by and the hydroxyl group in the carboxylic acid is the . This reaction causes the formation of the bond in the ester as well as the in the water:
The reaction forms an equilibrium that is established slowly unless a strong acid catalyst, such as concentrated , is used. This is not the only reaction used to form esters, but it is the most commonly used. It is called the Fischer esterification, and an example of this reaction is shown below. The reaction shows the combination of ethanoic acid and ethanol with an acid catalyst to produce ethyl ethanoate and water:
Ethanoic acid is also known as acetic acid, and ethyl ethanoate is also commonly referred to as ethyl acetate.
As water is produced from the combination of two molecules, esterification can be described as a condensation reaction.
Definition: Condensation Reaction (Dehydration Reaction)
Condensation or dehydration is a reaction that joins molecules in a chemical bond and results in the formation of a water molecule.
It should be noted that although the alcohol and carboxylic acid functional groups are usually on separate molecules, a cyclization reaction can occur if the functional groups are on the same molecule. This is most likely when the carbon chain between the alcohol and carboxylic acid groups is four or five carbons long. The reaction will produce the cyclic ester, lactone, and water.
Example 1: Predicting the Result of the Intramolecular Esterification of the 5-Hydroxypentanoic Acid
It is possible for a molecule containing an alcohol and a carboxylic acid group to react with itself to form cyclic esters known as lactones. Which structure would result from the reaction of 5-hydroxypentanoic acid with itself?
In this question, an esterification reaction is taking place; however, the alcohol and carboxylic acid involved in the reaction come from the same molecule.
The alkoxide group of the alcohol replaces the hydroxyl group of the carboxylic acid. Thus, a bond forms between and the adjacent to , and an ester link has been created. The reaction scheme for this reaction is as follows:
The five-carbon chain in the 5-hydroxypentanoic acid is linear with the functional groups at either end of the chain. Therefore, a six-membered lactone is produced—the six “members” include the five carbon atoms and the oxygen atom from the alcohol functional group.
It is evident that the product is therefore D.
Concentrated sulfuric acid is used as a catalyst for the reaction, but it is also used as a dehydrating agent to remove water and improve the yield of ester. Once the ester has been formed, it needs to be extracted from the reaction mixture, which will contain unreacted alcohol, carboxylic acid, and sulfuric acid.
The names of esters have two parts, which correspond to the alcohol and the carboxylic acid, or parent molecules, used to make them. To find out which molecules were used to form the ester, its bond must be split. The fragment containing is the carboxylic acid side.
Attaching a hydroxy group where the alkoxy group used to be will give the parent carboxylic acid. The alkoxy fragment is the alcohol side. Adding an to the will give the parent alcohol.
The diagram below shows how to find the parent carboxylic acid and alcohol for a general ester.
The diagram below shows how to find the parent carboxylic acid and alcohol for an example ester.
It is evident that the compounds used to make the ester were ethanoic acid, which is also commonly known as acetic acid, and methanol. The name of the parent molecules can be used to name the ester. The first part of the ester name comes from the alcohol name and involves changing the last part of the name to -yl. The second part comes from the name of the carboxylic acid and involves changing the last part of the name to -oate. Following on from the example shown above, the “methanol” becomes “methyl,” and the “ethanoic acid” becomes “ethanoate.” Therefore, the ester has the name “methyl ethanoate.” As ethanoic acid is also referred to as acetic acid, the ester may also be given the name methyl acetate. Note that, in this case, the suffix -ate is added rather than -oate.
The same naming pattern occurs for other alcohols when naming esters, for example, “ethanol” would become “ethyl,” and “propanol” would become “propyl.” If phenol is used to make an ester, then the ester’s name will include “phenyl.”
The case is similar for the carboxylic acids, just as “ethanoic acid” becomes “ethanoate” when naming esters, “methanoic acid,” “propanoic acid,” and “butanoic acid” would become “methanoate,” “propanoate,” and “butanoate.” It should also be noted that just as “ethanoic acid” is also called “acetic acid” and therefore “ethanoate” may also be “acetate,” “methanoic acid” is commonly called “formic acid;” thus, “methanoate” may also be called “formate.” For example, the ester methyl methanoate may also be given the name methyl formate.
If benzoic acid is used to make an ester, then the ester’s name will include “benzoate.”
Example 2: Naming an Ester from Its Structural Formula
Which of the following names does the ester shown have?
- Ethyl benzoate
- Phenyl benzoate
- Propyl benzoate
- Phenyl propanoate
- Benzyl methanoate
The names of esters have two parts, which correspond to their parent molecules: the alcohol and the carboxylic acid used to make them. The molecule must be split at the bond to separate the molecule into the carboxylic acid and alcohol parts. The fragment containing is the carboxylic acid side. Attaching a hydroxy group where the alkoxy group used to be will give the parent carboxylic acid. The propoxy fragment is the alcohol side. Adding an to the will give the parent alcohol.
The first part of the name comes from the alcohol and involves changing the last part of the name to -yl. The alcohol used to make the ester is propanol, as it contains three carbon atoms; thus, “propyl” is the first part of the ester’s name.
The second part comes from the name of the carboxylic acid and involves changing the last part of the name to -oate. The carboxylic acid used to make the ester was benzoic acid; thus, “benzoate” is the second part of the ester’s name.
Therefore, the answer to this question is C, propyl benzoate.
It should be noted that the name is written the opposite way round from how the condensed structural formula is written. The general formula of an ester is , where the first part comes from the parent carboxylic acid and the second part comes from the parent alcohol. The reverse is true when naming an ester. The first part of an ester’s name comes from the parent alcohol, and the second part comes from the parent carboxylic acid.
Example 3: Naming an Ester from Its Condensed Formula
What name, according to IUPAC nomenclature, does the ester have?
- Propyl ethanoate
- Ethyl ethanoate
- Ethoxy propanoxy
- Propyl propanoate
- Ethyl propanoate
The names of esters have two parts, which correspond to the alcohol and the carboxylic acid used to make them. The bond must be split to separate the molecule into the two parts. The left section of the formula (i.e., ) comes from the carboxylic acid, and the right section comes from the alcohol.
The first part of the name comes from the alcohol and involves changing the last part of the name to -yl. There are two carbon atoms in the alcohol section ; thus, ethanol was used to make the ester, and “ethyl” is the first part of the ester’s name.
The second part comes from the name of the carboxylic acid and involves changing the last part of the name to -oate. A common error when naming esters from condensed formulas is forgetting to count the carbon atom in the ester functional group . There are three carbon atoms in the carboxylic acid section ; thus, propanoic acid was used to make the ester, and “propanoate” is the second part of the ester’s name.
Therefore, it is evident that the answer is E, ethyl propanoate.
Not only will you need to be able to name an ester from its condensed formula, but also you will need to be able to predict the product of an esterification reaction.
Example 4: Predicting the Main Reaction Product of Ethanol and Butanoic Acid
Consider the following reaction:
What ester is produced from this reaction?
- Ethyl butanoate
- Ethyl propanoate
- Propyl butanoate
- Methyl butanoate
- Butyl ethanoate
Reacting an alcohol and a carboxylic acid produces an ester and water. The names of esters have two parts, which correspond to the alcohol and the carboxylic acid used to make them.
The first part of the name comes from the alcohol and involves changing the last part of the name to -yl. We are told that the alcohol used is ethanol; thus, the first part of the name is “ethyl.”
The second part comes from the name of the carboxylic acid and involves changing the last part of the name to -oate. We are told that the carboxylic acid used is butanoic acid; thus, the second part of the name is “butanoate.”
Therefore, the answer to this question is A, ethyl butanoate.
Most short-chain esters are sweet-smelling liquids, and they become less fragrant and more waxy when their molar mass is increased. Esters tend to have lower boiling points than alcohols and carboxylic acids of similar molar mass, as hydrogen bonds cannot form between ester molecules. This is because alcohols and carboxylic acids contain hydroxyl groups, but esters do not.
The following table shows two sets of three species that have different boiling points despite the fact that they have the same molar mass. The difference in their boiling points is due to their different hydrogen bonding abilities. Hydrogen bonds cannot form between ester molecules, but they can form between alcohol and carboxylic acid molecules. This is why methyl methanoate has a lower boiling point than propanol and ethanoic acid, and methyl ethanoate has a lower boiling point than butanol and propanoic acid.
|Species Type||Species Name||Condensed Structural Formula||Molar Mass (g/mol)||Boiling Point ()|
|Carboxylic acid||Ethanoic acid |
|Ester||Methyl methanoate |
|Methyl ethanoate |
A diagram showing hydrogen bonding in alcohols and carboxylic acids is shown below.
Lack of hydrogen bonding in esters is also the reason for their high volatility.
A volatile substance is one that evaporates readily at normal temperatures and pressures.
Esters can, however, form hydrogen bonds with water molecules, as shown below; thus, the smallest esters are soluble in water to an extent.
However, the solubility decreases with increasing chain length, and most esters are considered to be insoluble. Carbon chains are nonpolar and hydrophobic; thus, if an ester molecule has a long carbon chain, only a small proportion of the molecule will be able to form a hydrogen bond with water, making it insoluble.
Fats and oils are long-chain esters, which is why they are insoluble in water.
Example 5: Predicting the Longest Chain Length of Unknown Esters from Their Solubility
The table below shows the solubility of five unknown esters. Which of the unknown esters is likely to have the longest chain length? Assume the chains are linear and there is no branching.
|Solubility (g/100 mL)||8.7||0.6||10.5||2.2||4.9|
The solubility of esters decreases with increasing chain length as the carbon chain is nonpolar and hydrophobic, so the ester with the longest chain length will have the lowest solubility. 0.6 g/100 mL is the lowest value; thus, ester B is likely to have the longest chain length.
Therefore, the answer is D, ester B.
Esters with low molar masses are generally sweet smelling, but as their molar mass is increased, they become odorless. They are responsible for the smell and taste of many flowers and fruits and can also be found in pheromones. They are used as flavorings for food and drinks and in the production of perfumes. They are also commonly used as industrial solvents. The table below shows the different uses for a selection of esters.
|Propyl ethanoate |
|Methyl butanoate |
|Butyl ethanoate |
|Apple||Flavoring agent, |
nail polish solvent
|Ethyl ethanoate |
|Sweet||Solvent for the process of |
removing caffeine from
|Propyl methanoate |
|Apple, plum||Flavoring agent|
|2-Methylpropyl methanoate |
|Benzyl methanoate |
|Almond, apricot||Flavoring agent,|
|Pentyl ethanoate |
|Bananas, apples||Flavoring agent,|
|Methyl ethanoate |
|Sweet, glue-like||Solvent for glues, for paints,|
and in nail polish removers
Additionally, vegetable and animal oils and fats are primarily composed of natural tri-esters called triglycerides. Tri-esters are molecules containing three ester groups, and triglycerides are fatty acid esters of glycerol. The esterification reaction that produces triglycerides (oils and fats) is shown below:
The carbon chain in the fatty acids is typically between 4 and 24 carbon atoms long, is often linear with an even number of carbon atoms, and can be saturated or unsaturated. These fats and oils can be used to produce biodiesel.
Definition: Fatty Acid
A fatty acid is a monocarboxylic acid with a long aliphatic chain that is either saturated or unsaturated.
Polyesters are used as plastics and fibers, for example, the polyester poly(ethylene terephthalate) is used to make clothing fibers and plastic items such as water bottles. It also has applications in the medical industry where it is used in sutures, which are the threads used in stitches, and vascular grafts, which are used to replace damaged blood vessels.
Its formation is shown in the following reaction scheme:
Poly(ethylene terephthalate) can be abbreviated as PET or PETE. In the clothing industry, it is referred to as polyester or Dacron, and it can be identified on plastic items by a triangular symbol with a “1” in the middle, as shown.
In ancient civilizations, people used to chew on willow bark for pain relief, the active ingredient was found to be salicylic acid. Despite the benefits of salicylic acid, it causes adverse side effects; thus, scientists reacted it with ethanoic acid (acetic acid), producing an ester called acetylsalicylic acid of which the side effects are more minor. Acetylsalicylic acid is also known as aspirin and is used primarily as a painkiller. It can also be used as an antipyretic, which is a drug used to prevent or reduce fever. When taken daily in low doses, it can help prevent strokes and heart attacks. Unfortunately, taking aspirin slows the production of protective mucus stomach lining, so the stomach is more susceptible to damage from gastric acids, and stomach ulcers can form.
It is the phenolic part of salicylic acid that reacts to produce aspirin—it acts as the alcohol and reacts with a carboxylic acid to form the ester aspirin. Although phenols contain an group, so they will react, aliphatic alcohols react better.
However, since salicylic acid also contains a carboxylic acid functional group, it can also react with alcohols to form esters. An example of this would be its reaction with methanol to form oil of wintergreen (methyl salicylate), which also can be used to relieve pain and provide the minty flavor to chewing gum and mints. The reactions of salicylic acid to produce two different esters with different uses is shown below.
Let us summarize what we have learned about esters and their properties.
- Ester molecules are organic compounds represented by the general formula and are formed by replacing a hydroxyl group in an acid with an alkoxy group.
- The most common method of forming esters involves the (condensation) reaction of an alcohol and a carboxylic acid.
- The names of esters come from the names of the alcohols and carboxylic acids used to make them. The first part of the name comes from the alcohol, but the ending of the alcohol’s name is changed to -yl, and the second part of the name comes from the carboxylic acid, but the ending of the carboxylic acid’s name is changed to -oate or -ate.
- Esters tend to have lower boiling points than alcohols and carboxylic acids as hydrogen bonds cannot form between ester molecules.
- Esters are volatile and are largely insoluble.
- Esters tend to be sweet smelling; thus, they are used as flavorings and in perfumes. They are also used as industrial solvents, used to make up vegetable and animal oils and fats, and can be polymerized to produce polyesters. They are also used in medicine such as aspirin and oil of wintergreen.